1. Field of the Invention
The present invention relates to a resistance adjusting type heater and a catalytic converter which are suitable for use in automotive exhaust emissions control.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as catalysts or substrates for catalyst which remove, for example, nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbon (HC) present in the exhaust gas of internal combustion engines, such as automobiles. Recently, however, metal honeycomb structures have been drawing attention as such catalysts or substrates therefor.
Apart from the above catalysts or catalyst substrates, as restrictions on exhaust emissions have been intensified, there has been a demand for development of heaters or catalytic converters capable of converting the emissions of an engine when it is started.
Such honeycomb structures have been proposed in, for example, Japanese Utility Model Laid-Open No. 67609/1988. This disclosed technique is a catalytic converter comprising a metal monolithic catalyst which is disposed upstream of and adjacent to a main ceramic monolithic catalyst. The metal monolithic catalyst comprises an electrically conductible metal substrate with alumina coated thereon.
However, in the catalytic converter disclosed in Japanese Utility Model Laid-Open No. 67609/1988, the metal monolithic catalyst, a preheater, disposed upstream and adjacent to the main monolithic catalyst comprises a foil-type metal honeycomb structure which is constructed such that it is conducted and thereby heated from the inner periphery thereof to the outer periphery thereof. The resistance of the metal monolithic catalyst is not adjusted (i.e., only the material, dimensions and rib thickness of the metal honeycomb structure are defined and no adjustment is made on the resistance of the metal honeycomb structure). Furthermore, since the diameter of the metal monolithic catalyst is substantially the same as that of the main monolithic catalyst, the catalytic converter exhibits insufficient temperature rising characteristics.